Abstract: The precipitation efficiency of stratiform clouds is usually low, and thus stratiform clouds have a high potential for the development and utilization of cloud water resources. They are important targets for artificial precipitation enhancement. With the increasing demands for ecological improvement, reservoir storage, drought resistance, and other social needs in southern China, it is increasingly necessary to study the technology and mechanism of cloud seeding in stratiform cloud precipitation system over this region. In this paper, a three-dimensional mesoscale cold cloud seeding model was used to simulate the stratiform cloud system rainfall process and silver iodide (AgI) seeding operation by aircraft for rainfall enhancement in Hubei province, China on 21 October 2018. The main macro and micro characteristics of clouds and rainfall were reasonably simulated. Both observational data and simulation results show favorable cloud condition for AgI seeding in the aircraft operation area. On this basis, the entire aircraft seeding process along the real flight trajectory and seeding information were simulated. The numerical simulation results show that condensation-freezing nucleation and deposition nucleation are the main nucleation modes of AgI during the seeding operation. The local activation ratio of ice nuclei ranged from 0.01% to 2% for more than 90% of AgI, and the average value of AgI activation ratio was between 0.07% and 0.27%. Rainfall was the result of joint effects of cold cloud precipitation mechanism and warm cloud precipitation mechanism, and seeding operation enhanced the effects of the two precipitation mechanisms and achieved obvious rainfall enhancement effect. Four hours after the start of the seeding operation, the accumulated rainfall in the entire evaluation area increased by 2.12×10⁸ kg, which was equivalent to 8.1% of regional total rainfall; on local scale, the rainfall enhancement by seeding varied between −51.1% and 306.7%. Due to the dynamic disturbances caused by seeding in the supercooled cloud region, updrafts were weakened in some areas. As a result, the growth of precipitation particles was weakened and rainfall reduced in these areas. Seeding AgI in supercooled cloud region could lead to high concentration of the ice crystal particles, and the conversion processes from ice crystals to snow and from snow to graupels were enhanced. Following the increases in snow and graupel particles, more snow and graupel particles fell into the warm region of the clouds, and more large raindrops were generated in the upper layer of the warm region, which enhanced the collision-coalescence process between raindrops and cloud droplets in the warm region, and eventually led to rainfall increases. The above processes formed a main microphysical chain of rainfall enhancement by AgI seeding.